1. Field of the Invention
The disclosure provides a new photopolymerizable resin system for dental restorative materials. The resin system utilizes a thiol-ene component as the reactive diluent in dimethacrylate systems. The ternary resin system comprises a thiol monomer, an ene monomer and a dimethacrylate monomer. Use of an off-stoichiometric ratio of thiol:ene functional groups in favor of excess thiols results in enhanced overall functional group conversion, improved polymer mechanical properties, and reduced shrinkage stress of the ternary system when compared to either traditional dimethacrylate or thiol-ene resin systems.
2. Background Art
Currently, most commercial photocurable dental restorative resins are based on dimethacrylates and the reaction mechanism is through chain-growth free radical polymerization. Existing dimethacrylate systems are popular for fillings and other dental prostheses because of their esthetic merit and “cure-on-command” feature. These formulations have resulted in significant advancements in the field of dentistry.
Such dental restorative materials are often mixed with 45 to 85% by weight (wt %) silanized filler compounds such as barium, strontium, zirconia silicate and/or amorphous silica to match the color and opacity to a particular use or tooth. The filler is typically in the form of particles with a size ranging from 0.01 to 5.0 micrometers.
The photocurable restorative materials are often sold in separate syringes or single-dose capsules of different shades. If provided in a syringe, the user dispenses (by pressing a plunger or turning a screw adapted plunger on the syringe) the necessary amount of restorative material from the syringe. Then the material is placed directly into the cavity, mold, or location of use. If provided as a single-dose capsule, the capsule is placed into a dispensing device that can dispense the material directly into the cavity, mold, etc. After the restorative material is placed, it is photopolymerized or cured by exposing the restorative material to the appropriate light source. The resulting cured polymer may then be finished or polished as necessary with appropriate tools. Such dental restoratives can be used for direct anterior and posterior restorations, core build-ups, splinting and indirect restorations including inlays, onlays and veneers.
Although easy to use, these dimethacrylate systems have several drawbacks and there are a number of properties of the resin chemistry that, if improved upon, would increase the performance, longevity and biocompatibility of composite restorations (Sakaguchi et al., Dental Materials 21:43-46, 2005; Dauvillier et al., Journal of Biomedical Materials Research 58(1):16-26, 2001; Dauvillier et al., Journal of Dental Research 79(3):818-823, 2000; Yourtee et al., In Vitro Toxicology 10:245-251, 1997). The most significant shortcomings of methacrylate-based resins are high volumetric shrinkage (Ferracane, Dental Materials 21:36-42, 2005), high polymerization stress (Braga et al., Dental Materials 21:962-970, 2005; Lu et al., Dental Materials, 21(12):1129-1136, 2005; Braga and Ferracane, Journal of Dental Research 81:114-118, 2002) and low functional group conversion (Darmani and Al-Hiyasat, Dental Materials 22:353-358, 2006; Sasaki et al., Journal of Materials Science: Materials in Medicine 16:297-300, 2005; Pulgar et al., Environmental Health Perspectives 108:21-27, 2000). The chain growth polymerization mechanism results in long chains and therefore early gelation which contributes to both volume shrinkage and shrinkage stress. The current systems typically only reach a final double bond conversion of 55 to 75%, which not only contributes to the insufficient wear resistance and mechanical properties, but also jeopardizes the biocompatibility of the composites due to the leachable unreacted monomers. Additionally, the residual monomer left in the restoration after curing is extractable and may leach out of the restoration and into the body, with unknown consequences (Sasaki et al., 2005; Pulgar et al., 2000). There is concern that residual monomers may cause allergic reactions and sensitization in patients (Theilig et al., Journal of Biomedical Materials Research 53(6):632-639, 2000). There is also reason to believe that release of the most common reactive diluent, triethylene glycol dimethacrylate (TEGDMA), may also contribute to local and systemic adverse effects by dental composites (Hansel et al., Journal of Dental Research 77(1):60-67, 1998; Englemann et al., Journal of Dental Research 80(3):869-875, 2001; Schweikl and Schmalz, Mutation Research—Genetic Toxicology and Environmental Mutagenesis 438:71-78, 1999; Darmani and Al-Hiyasat, 2006).
Upon polymerization, shrinkage stresses transferred to the tooth can cause deformation of the cusp or enamel microcracks (Davidson and Feilzer, J Dent. 25:435-440, 1997; Suliman et al., Journal of Dental Research 72(11): 1532-1536, 1993a; Suliman et al., Dental Materials 9(1):6-10, 1993b), and stress at the tooth-composite interface may cause adhesive failure, initiation of microleakage and recurrent caries. In addition, significant increases in volumetric shrinkage and shrinkage stress are experienced when the double bond conversion is increased to reduce the leachable monomer (Lu et al., Journal of Biomedical Materials Research Part B—Applied Biomaterials, 71B:206-213, 2004). This trade-off of conversion and shrinkage has been an inherent problem with composite restorative materials since their inception.
Recently, thiol-enes have been investigated as alternatives to dimethacrylate dental restorative materials (Lu et al., 2005; Cramer et al., “Investigation of Thiol-Ene Based Systems as Dental Restorative Materials” to be submitted to Dental Materials. 2009). The reactions proceed via a step growth addition mechanism that comprises the addition of a thiyl radical through a vinyl functional group and subsequent chain transfer to a thiol, regenerating the thiyl radical (Jacobine, A. F. Radiation Curing in Polymer Science and Technology III, Polymerisation Mechanisms; Fouassier, J. D.; Rabek, J. F., Ed.; Elsevier Applied Science, London, 1993; Chapter 7, 219; Hoyle et al., Journal of polymer Science: Part A: Polymer Chemistry, 2004, 42, 5301-5338; Cramer and Bowman, Journal of Polymer Science. Part A. Polymer Chemistry, 2001, 39 (19), 3311; Cramer et al., Macromolecules, 2003a, 36 (12), 4631; Cramer et al., Macromolecules, 2003b, 36 (21), 7964; Reddy et al. Macromolecules, 2006, 39(10), 3673). The step-growth polymerization mechanism results in shorter polymer chains and delayed gelation, resulting in reduced volume shrinkage and shrinkage stress. It is well known that in thiol-ene step growth polymerizations, the thiol and ene components must be present in a 1:1 stoichiometric ratio of functional groups to achieve complete conversion and maximize polymer mechanical properties (Morgan et al., J. Polym. Sci., A, Polym. Chem. 627, 1977; Jacobine et al., Journal of Applied Polymer Science 45(3):471-485, 1992; Cramer and Bowman, 2001; Hoyle et al., 2004). The high functional group conversion of thiol-ene polymers significantly mitigates the problems associated with current dimethacrylate resin systems which are associated with incomplete double bond conversion. Besides the impact of the polymerization mechanism on the gel point conversion and network formation, the thiol-ene systems have advantageous curing kinetics demonstrating rapid polymerization rates, high overall functional group conversion, and little sensitivity to oxygen inhibition (Lu et al., Dental Materials, 21(12), 2005, 1129-1136; Cramer et al., Macromolecules, 35, 5361, 2002; Hoyle et al., 2004).
Most importantly for dental restorative materials, thiol-enes exhibit reduced shrinkage and shrinkage stress due to the step growth mechanism and delayed gel point conversion (Chiou et al., Macromolecules, 1997, 30, 7322; Lu et al., 2005). As a result of the delayed gel point, much of the shrinkage occurs before gelation, which dramatically reduces the shrinkage stress in the final polymer material.
The thiol-ene polymerization has also demonstrated thicker curing depth than methacrylate based resin systems. This can reduce the patient's chair-time since one-step curing is feasible, especially for large cavity filling, where incremental filling has to be applied using current dental composite systems. In addition, the thick cure depth and lack of oxygen inhibition of thiol-ene systems leads to fewer filling and curing steps during restorations, compared with the incremental filling technique using current dimethacrylate dental resin systems
Unfortunately, despite several advantages of the thiol-ene resin systems for use as dental restorative materials, previous studies have also shown that traditional binary thiol-ene systems exhibit mechanical properties that are not ideal; specifically low flexural modulus and strength relative to dimethacrylate resins (Lu et al., 2005; Cramer et al., 2009). Thus, it is important to develop rapidly curing dental restorative materials with improved monomer conversion and mechanical properties, while concurrently reducing volumetric shrinkage and shrinkage stress.